constraint

Use a constraint to define an integrity constraint—a rule that restricts the values in a database. Oracle Database lets you create six types of constraints and lets you declare them in two ways.

The six types of integrity constraint are described briefly here and more fully in "Semantics ":

A NOTNULLconstraint prohibits a database value from being null.

A unique constraint prohibits multiple rows from having the same value in the same column or combination of columns but allows some values to be null.

A primary key constraint combines a NOTNULL constraint and a unique constraint in a single declaration. That is, it prohibits multiple rows from having the same value in the same column or combination of columns and prohibits values from being null.

A foreign key constraint requires values in one table to match values in another table.

A check constraint requires a value in the database to comply with a specified condition.

A REF column by definition references an object in another object type or in a relational table. A REF constraint lets you further describe the relationship between the REF column and the object it references.

You can define constraints syntactically in two ways:

As part of the definition of an individual column or attribute. This is called inline specification.

As part of the table definition. This is called out-of-line specification.

NOTNULL constraints must be declared inline. All other constraints can be declared either inline or out of line.

Oracle Database does not enforce view constraints. However, you can enforce constraints on views through constraints on base tables.

You can specify only unique, primary key, and foreign key constraints on views, and they are supported only in DISABLE NOVALIDATE mode. You cannot define view constraints on attributes of an object column.

You must have the privileges necessary to issue the statement in which you are defining the constraint.

To create a foreign key constraint, in addition, the parent table or view must be in your own schema or you must have the REFERENCES privilege on the columns of the referenced key in the parent table or view.

This section describes the semantics of constraint. For additional information, refer to the SQL statement in which you define or redefine a constraint for a table or view.

Oracle Database does not support constraints on columns or attributes whose type is a user-defined object, nested table, VARRAY, REF, or LOB, with two exceptions:

NOTNULL constraints are supported for a column or attribute whose type is user-defined object, VARRAY, REF, or LOB.

NOTNULL, foreign key, and REF constraints are supported on a column of type REF.

CONSTRAINT constraint_name

Specify a name for the constraint. If you omit this identifier, then Oracle Database generates a name with the form SYS_Cn. Oracle stores the name and the definition of the integrity constraint in the USER_, ALL_, and DBA_CONSTRAINTS data dictionary views (in the CONSTRAINT_NAME and SEARCH_CONDITION columns, respectively).

A NOTNULL constraint prohibits a column from containing nulls. The NULL keyword by itself does not actually define an integrity constraint, but you can specify it to explicitly permit a column to contain nulls. You must define NOTNULL and NULL using inline specification. If you specify neither NOTNULL nor NULL, then the default is NULL.

NOTNULL constraints are the only constraints you can specify inline on XMLType and VARRAY columns.

To satisfy a NOTNULL constraint, every row in the table must contain a value for the column.

Note:

Oracle Database does not index table rows in which all key columns are null except in the case of bitmap indexes. Therefore, if you want an index on all rows of a table, then you must either specify NOTNULL constraints for at least one of the index key columns or create a bitmap index.

Restrictions on NOT NULL Constraints

You cannot specify NULL or NOTNULL in a view constraint.

You cannot specify NULL or NOTNULL for an attribute of an object. Instead, use a CHECK constraint with the IS [NOT] NULL condition.

A unique constraint designates a column as a unique key. A composite unique key designates a combination of columns as the unique key. When you define a unique constraint inline, you need only the UNIQUE keyword. When you define a unique constraint out of line, you must also specify one or more columns. You must define a composite unique key out of line.

To satisfy a unique constraint, no two rows in the table can have the same value for the unique key. However, the unique key made up of a single column can contain nulls. To satisfy a composite unique key, no two rows in the table or view can have the same combination of values in the key columns. Any row that contains nulls in all key columns automatically satisfies the constraint. However, two rows that contain nulls for one or more key columns and the same combination of values for the other key columns violate the constraint.

When you specify a unique constraint on one or more columns, Oracle implicitly creates an index on the unique key. If you are defining uniqueness for purposes of query performance, then Oracle recommends that you instead create the unique index explicitly using a CREATEUNIQUEINDEX statement. See CREATE INDEX for more information.

Restrictions on Unique Constraints

None of the columns in the unique key can be of LOB, LONG, LONGRAW, VARRAY, NESTEDTABLE, OBJECT, REF, TIMESTAMPWITHTIMEZONE, or user-defined type. However, the unique key can contain a column of TIMESTAMPWITHLOCALTIMEZONE.

A composite unique key cannot have more than 32 columns.

You cannot designate the same column or combination of columns as both a primary key and a unique key.

You cannot specify a unique key when creating a subview in an inheritance hierarchy. The unique key can be specified only for the top-level (root) view.

A primary key constraint designates a column as the primary key of a table or view. A composite primary key designates a combination of columns as the primary key. When you define a primary key constraint inline, you need only the PRIMARYKEY keywords. When you define a primary key constraint out of line, you must also specify one or more columns. You must define a composite primary key out of line.

A primary key constraint combines a NOTNULL and unique constraint in one declaration. Therefore, to satisfy a primary key constraint:

No primary key value can appear in more than one row in the table.

No column that is part of the primary key can contain a null.

Restrictions on Primary Key Constraints

A table or view can have only one primary key.

None of the columns in the primary key can be LOB, LONG, LONGRAW, VARRAY, NESTEDTABLE, BFILE, REF, TIMESTAMPWITHTIMEZONE, or user-defined type. However, the primary key can contain a column of TIMESTAMPWITHLOCALTIMEZONE.

The size of the primary key cannot exceed approximately one database block.

A composite primary key cannot have more than 32 columns.

You cannot designate the same column or combination of columns as both a primary key and a unique key.

You cannot specify a primary key when creating a subview in an inheritance hierarchy. The primary key can be specified only for the top-level (root) view.

A foreign key constraint (also called a referential integrity constraint) designates a column as the foreign key and establishes a relationship between that foreign key and a specified primary or unique key, called the referenced key. A composite foreign key designates a combination of columns as the foreign key.

The table or view containing the foreign key is called the child object, and the table or view containing the referenced key is called the parent object. The foreign key and the referenced key can be in the same table or view. In this case, the parent and child tables are the same. If you identify only the parent table or view and omit the column name, then the foreign key automatically references the primary key of the parent table or view. The corresponding column or columns of the foreign key and the referenced key must match in order and datatype.

You can define a foreign key constraint on a single key column either inline or out of line. You must specify a composite foreign key and a foreign key on an attribute out of line.

To satisfy a composite foreign key constraint, the composite foreign key must refer to a composite unique key or a composite primary key in the parent table or view, or the value of at least one of the columns of the foreign key must be null.

You can designate the same column or combination of columns as both a foreign key and a primary or unique key. You can also designate the same column or combination of columns as both a foreign key and a cluster key.

You can define multiple foreign keys in a table or view. Also, a single column can be part of more than one foreign key.

Restrictions on Foreign Key Constraints

None of the columns in the foreign key can be of LOB, LONG, LONGRAW, VARRAY, NESTEDTABLE, BFILE, REF, TIMESTAMPWITHTIMEZONE, or user-defined type. However, the primary key can contain a column of TIMESTAMPWITHLOCALTIMEZONE.

The referenced unique or primary key constraint on the parent table or view must already be defined.

A composite foreign key cannot have more than 32 columns.

The child and parent tables must be on the same database. To enable referential integrity constraints across nodes of a distributed database, you must use database triggers. See CREATE TRIGGER .

If either the child or parent object is a view, then the constraint is subject to all restrictions on view constraints. See "View Constraints".

You cannot define a foreign key constraint in a CREATETABLE statement that contains an ASsubquery clause. Instead, you must create the table without the constraint and then add it later with an ALTERTABLE statement.

Foreign key constraints use the references_clause syntax. When you specify a foreign key constraint inline, you need only the references_clause. When you specify a foreign key constraint out of line, you must also specify the FOREIGNKEY keywords and one or more columns.

ON DELETE Clause

The ONDELETE clause lets you determine how Oracle Database automatically maintains referential integrity if you remove a referenced primary or unique key value. If you omit this clause, then Oracle does not allow you to delete referenced key values in the parent table that have dependent rows in the child table.

A check constraint lets you specify a condition that each row in the table must satisfy. To satisfy the constraint, each row in the table must make the condition either TRUE or unknown (due to a null). When Oracle evaluates a check constraint condition for a particular row, any column names in the condition refer to the column values in that row.

The syntax for inline and out-of-line specification of check constraints is the same. However, inline specification can refer only to the column (or the attributes of the column if it is an object column) currently being defined, whereas out-of-line specification can refer to multiple columns or attributes.

Oracle does not verify that conditions of check constraints are not mutually exclusive. Therefore, if you create multiple check constraints for a column, design them carefully so their purposes do not conflict. Do not assume any particular order of evaluation of the conditions.

You cannot specify a check constraint for a view. However, you can define the view using the WITHCHECKOPTION clause, which is equivalent to specifying a check constraint for the view.

The condition of a check constraint can refer to any column in the table, but it cannot refer to columns of other tables.

Conditions of check constraints cannot contain the following constructs:

Subqueries and scalar subquery expressions

Calls to the functions that are not deterministic (CURRENT_DATE, CURRENT_TIMESTAMP, DBTIMEZONE, LOCALTIMESTAMP, SESSIONTIMEZONE, SYSDATE, SYSTIMESTAMP, UID, USER, and USERENV)

Calls to user-defined functions

Dereferencing of REF columns (for example, using the DEREF function)

Nested table columns or attributes

The pseudocolumns CURRVAL, NEXTVAL, LEVEL, or ROWNUM

Date constants that are not fully specified

REF Constraints

REF constraints let you describe the relationship between a column of type REF and the object it references.

ref_constraint

REF constraints use the ref_constraint syntax. You define a REF constraint either inline or out of line. Out-of-line specification requires you to specify the REF column or attribute you are further describing.

For ref_column, specify the name of a REF column of an object or relational table.

For ref_attribute, specify an embedded REF attribute within an object column of a relational table.

Both inline and out-of-line specification let you define a scope constraint, a rowid constraint, or a referential integrity constraint on a REF column.

If the scope table or referenced table of the REF column has a primary-key-based object identifier, then the REF column is a user-definedREFcolumn.

In a table with a REF column, each REF value in the column can conceivably reference a row in a different object table. The SCOPE clause restricts the scope of references to a single table, scope_table. The values in the REF column or attribute point to objects in scope_table, in which object instances of the same type as the REF column are stored.

Specify the SCOPE clause to restrict the scope of references in the REF column to a single table. For you to specify this clause, scope_table must be in your own schema or you must have SELECT privileges on scope_table or SELECTANYTABLE system privileges. You can specify only one scope table for each REF column.

Restrictions on Scope Constraints

You cannot add a scope constraint to an existing column unless the table is empty.

You cannot specify a scope constraint for the REF elements of a VARRAY column.

You must specify this clause if you specify ASsubquery and the subquery returns user-defined REFs.

You cannot subsequently drop a scope constraint from a REF column.

Rowid REF Constraints

Specify WITHROWID to store the rowid along with the REF value in ref_column or ref_attribute. Storing the rowid with the REF value can improve the performance of dereferencing operations, but will also use more space. Default storage of REF values is without rowids.

You cannot define a rowid constraint for the REF elements of a VARRAY column.

You cannot subsequently drop a rowid constraint from a REF column.

If the REF column or attribute is scoped, then this clause is ignored and the rowid is not stored with the REF value.

Referential Integrity Constraints on REF Columns

The references_clause of the ref_constraint syntax lets you define a foreign key constraint on the REF column. This clause also implicitly restricts the scope of the REF column or attribute to the referenced table. However, whereas a foreign key constraint on a non-REF column references an actual column in the parent table, a foreign key constraint on a REF column references the implicit object identifier (OID) column of the parent table.

If you do not specify a constraint name, then Oracle generates a system name for the constraint of the form SYS_Cn.

If you add a referential integrity constraint to an existing REF column that is already scoped, then the referenced table must be the same as the scope table of the REF column. If you later drop the referential integrity constraint, then the REF column will remain scoped to the referenced table.

As is the case for foreign key constraints on other types of columns, you can use the references_clause alone for inline declaration. For out-of-line declaration you must also specify the FOREIGNKEY keywords plus one or more REF columns or attributes.

Oracle implicitly adds a scope constraint when you add a referential integrity constraint to an existing unscoped REF column. Therefore, all the restrictions that apply for scope constraints also apply in this case.

You cannot specify a column after the object name in the references_clause.

Specifying Constraint State

As part of constraint definition, you can specify how and when Oracle should enforce the constraint.

constraint_state

You can use the constraint_state with both inline and out-of-line specification. You can specify the clauses of constraint_state in any order, but you can specify each clause only once.

DEFERRABLE Clause

The DEFERRABLE and NOTDEFERRABLE parameters indicate whether or not, in subsequent transactions, constraint checking can be deferred until the end of the transaction using the SETCONSTRAINT(S) statement. If you omit this clause, then the default is NOTDEFERRABLE.

Specify NOTDEFERRABLE to indicate that in subsequent transactions you cannot use the SETCONSTRAINT[S] clause to defer checking of this constraint until the transaction is committed. The checking of a NOTDEFERRABLE constraint can never be deferred to the end of the transaction.

If you declare a new constraint NOTDEFERRABLE, then it must be valid at the time the CREATETABLE or ALTERTABLE statement is committed or the statement will fail.

Specify DEFERRABLE to indicate that in subsequent transactions you can use the SETCONSTRAINT[S] clause to defer checking of this constraint until after the transaction is committed. This setting in effect lets you disable the constraint temporarily while making changes to the database that might violate the constraint until all the changes are complete.

You cannot alter the deferrability of a constraint. That is, whether you specify either of these parameters, or make the constraint NOTDEFERRABLE implicitly by specifying neither of them, you cannot specify this clause in an ALTERTABLE statement. You must drop the constraint and re-create it.

The INITIALLY clause establishes the default checking behavior for constraints that are DEFERRABLE. The INITIALLY setting can be overridden by a SETCONSTRAINT(S) statement in a subsequent transaction.

Specify INITIALLYIMMEDIATE to indicate that Oracle should check this constraint at the end of each subsequent SQL statement. If you do not specify INITIALLY at all, then the default is INITIALLYIMMEDIATE.

If you declare a new constraint INITIALLYIMMEDIATE, then it must be valid at the time the CREATETABLE or ALTERTABLE statement is committed or the statement will fail.

Specify INITIALLYDEFERRED to indicate that Oracle should check this constraint at the end of subsequent transactions.

This clause is not valid if you have declared the constraint to be NOTDEFERRABLE, because a NOTDEFERRABLE constraint is automatically INITIALLYIMMEDIATE and cannot ever be INITIALLYDEFERRED.

VALIDATE | NOVALIDATE

The behavior of VALIDATE and NOVALIDATE always depends on whether the constraint is enabled or disabled, either explicitly or by default. Therefore they are described in the context of "ENABLE Clause" and "DISABLE Clause".

ENABLE Clause

Specify ENABLE if you want the constraint to be applied to the data in the table.

If you enable a unique or primary key constraint, and if no index exists on the key, then Oracle Database creates a unique index. Unless you specify KEEP INDEX when subsequently disabling the constraint, this index is dropped and the database rebuilds the index every time the constraint is reenabled.

You can also avoid rebuilding the index and eliminate redundant indexes by creating new primary key and unique constraints initially disabled. Then create (or use existing) nonunique indexes to enforce the constraint. Oracle does not drop a nonunique index when the constraint is disabled, so subsequent ENABLE operations are facilitated.

ENABLEVALIDATE specifies that all old and new data also complies with the constraint. An enabled validated constraint guarantees that all data is and will continue to be valid.

If any row in the table violates the integrity constraint, the constraint remains disabled and Oracle returns an error. If all rows comply with the constraint, Oracle enables the constraint. Subsequently, if new data violates the constraint, Oracle does not execute the statement and returns an error indicating the integrity constraint violation.

If you place a primary key constraint in ENABLEVALIDATE mode, the validation process will verify that the primary key columns contain no nulls. To avoid this overhead, mark each column in the primary key NOTNULL before entering data into the column and before enabling the primary key constraint of the table.

ENABLENOVALIDATE ensures that all new DML operations on the constrained data comply with the constraint. This clause does not ensure that existing data in the table complies with the constraint and therefore does not require a table lock.

If you specify neither VALIDATE nor NOVALIDATE, the default is VALIDATE.

If you change the state of any single constraint from ENABLENOVALIDATE to ENABLEVALIDATE, the operation can be performed in parallel, and does not block reads, writes, or other DDL operations.

Restriction on the ENABLE Clause

You cannot enable a foreign key that references a disabled unique or primary key.

DISABLE Clause

Specify DISABLE to disable the integrity constraint. Disabled integrity constraints appear in the data dictionary along with enabled constraints. If you do not specify this clause when creating a constraint, Oracle automatically enables the constraint.

DISABLEVALIDATE disables the constraint and drops the index on the constraint, but keeps the constraint valid. This feature is most useful in data warehousing situations, because it lets you load large amounts of data while also saving space by not having an index. This setting lets you load data from a nonpartitioned table into a partitioned table using the exchange_partition_clause of the ALTERTABLE statement or using SQL*Loader. All other modifications to the table (inserts, updates, and deletes) by other SQL statements are disallowed.

DISABLENOVALIDATE signifies that Oracle makes no effort to maintain the constraint (because it is disabled) and cannot guarantee that the constraint is true (because it is not being validated).

You cannot drop a table whose primary key is being referenced by a foreign key even if the foreign key constraint is in DISABLENOVALIDATE state. Further, the optimizer can use constraints in DISABLENOVALIDATE state.

If you specify neither VALIDATE nor NOVALIDATE, then the default is NOVALIDATE.

If you disable a unique or primary key constraint that is using a unique index, then Oracle drops the unique index. Please refer to the CREATETABLEenable_disable_clause for additional notes and restrictions.

RELY Clause

RELY and NORELY are valid only when you are modifying an existing constraint (that is, in the ALTERTABLE ... MODIFY constraint syntax). These parameters specify whether a constraint in NOVALIDATE mode is to be taken into account for query rewrite. Specify RELY to activate an existing constraint in NOVALIDATE mode for query rewrite in an unenforced query rewrite integrity mode. The constraint is in NOVALIDATE mode, so Oracle does not enforce it. The default is NORELY.

Unenforced constraints are generally useful only with materialized views and query rewrite. Depending on the QUERY_REWRITE_INTEGRITY mode, query rewrite can use only constraints that are in VALIDATE mode, or that are in NOVALIDATE mode with the RELY parameter set, to determine join information.

When defining the state of a unique or primary key constraint, you can specify an index for Oracle to use to enforce the constraint, or you can instruct Oracle to create the index used to enforce the constraint.

using_index_clause

You can specify the using_index_clause only when enabling unique or primary key constraints. You can specify the clauses of the using_index_clause in any order, but you can specify each clause only once.

If you specify schema.index, then Oracle attempts to enforce the constraint using the specified index. If Oracle cannot find the index or cannot use the index to enforce the constraint, then Oracle returns an error.

If you specify the create_index_statement, then Oracle attempts to create the index and use it to enforce the constraint. If Oracle cannot create the index or cannot use the index to enforce the constraint, then Oracle returns an error.

If you neither specify an existing index nor create a new index, then Oracle creates the index. In this case:

The index receives the same name as the constraint.

If table is partitioned, then you can specify a locally or globally partitioned index for the unique or primary key constraint.

Restrictions on the using_index_clause

You cannot specify this clause for a view constraint.

You cannot specify this clause for a NOTNULL, foreign key, or check constraint.

You cannot specify an index (schema.index) or create an index (create_index_statement) when enabling the primary key of an index-organized table.

You cannot specify the domain_index_clause of index_properties or the parallel_clause of index_attributes.

When defining the state of a constraint, you can specify a table into which Oracle places the rowids of all rows violating the constraint.

exceptions_clause

Use the exceptions_clause syntax to define exception handling. If you omit schema, then Oracle assumes the exceptions table is in your own schema. If you omit this clause altogether, then Oracle assumes that the table is named EXCEPTIONS. The EXCEPTIONS table or the table you specify must exist on your local database.

You can create the EXCEPTIONS table using one of these scripts:

UTLEXCPT.SQL uses physical rowids. Therefore it can accommodate rows from conventional tables but not from index-organized tables. (See the Note that follows.)

UTLEXPT1.SQL uses universal rowids, so it can accommodate rows from both conventional and index-organized tables.

If you create your own exceptions table, then it must follow the format prescribed by one of these two scripts.

If you are collecting exceptions from index-organized tables based on primary keys (rather than universal rowids), then you must create a separate exceptions table for each index-organized table to accommodate its primary-key storage. You create multiple exceptions tables with different names by modifying and resubmitting the script.

Restrictions on the exceptions_clause

You cannot specify this clause for a view constraint.

You cannot specify this clause in a CREATETABLE statement, because no rowids exist until after the successful completion of the statement.

Oracle does not enforce view constraints. However, operations on views are subject to the integrity constraints defined on the underlying base tables. This means that you can enforce constraints on views through constraints on base tables.

Restrictions on View Constraints

View constraints are a subset of table constraints and are subject to the following restrictions:

You can specify only unique, primary key, and foreign key constraints on views. However, you can define the view using the WITHCHECKOPTION clause, which is equivalent to specifying a check constraint for the view.

Because view constraints are not enforced directly, you cannot specify INITIALLYDEFERRED or DEFERRABLE.

View constraints are supported only in DISABLENOVALIDATE mode. You must specify the keywords DISABLENOVALIDATE when you declare the view constraint, and you cannot specify any other mode.

You cannot specify the using_index_clause, the exceptions_clause clause, or the ONDELETE clause of the references_clause.

You cannot define view constraints on attributes of an object column.

Examples

Unique Key Example

The following statement is a variation of the statement that created the sample table sh.promotions. It defines inline and implicitly enables a unique key on the promo_id column (other constraints are not shown):

The constraint promo_id_u identifies the promo_id column as a unique key. This constraint ensures that no two promotions in the table have the same ID. However, the constraint does allow promotions without identifiers.

The wh_unq constraint ensures that the same combination of warehouse_id and warehouse_name values does not appear in the table more than once.

The ADDCONSTRAINT clause also specifies other properties of the constraint:

The USINGINDEX clause specifies storage characteristics for the index Oracle creates to enable the constraint.

The EXCEPTIONSINTO clause causes Oracle to write to the wrong_id table information about any rows currently in the warehouses table that violate the constraint. If the wrong_id exceptions table does not already exist, then this statement will fail.

Primary Key Example

The following statement is a variation of the statement that created the sample table hr.locations. It creates the locations_demo table and defines and enables a primary key on the location_id column (other constraints from the hr.locations table are omitted):

The loc_id_pk constraint, specified inline, identifies the location_id column as the primary key of the locations_demo table. This constraint ensures that no two locations in the table have the same location number and that no location identifier is NULL.

This constraint identifies the combination of the prod_id and cust_id columns as the primary key of the sales table. The constraint ensures that no two rows in the table have the same combination of values for the prod_id column and cust_id columns.

The constraint clause (PRIMARYKEY) also specifies the following properties of the constraint:

The constraint definition does not include a constraint name, so Oracle generates a name for the constraint.

The DISABLE clause causes Oracle to define the constraint but not enable it.

Foreign Key Constraint Example

The following statement creates the dept_20 table and defines and enables a foreign key on the department_id column that references the primary key on the department_id column of the departments table:

The constraint fk_deptno ensures that all departments given for employees in the dept_20 table are present in the departments table. However, employees can have null department numbers, meaning they are not assigned to any department. To ensure that all employees are assigned to a department, you could create a NOTNULL constraint on the department_id column in the dept_20 table in addition to the REFERENCES constraint.

Before you define and enable this constraint, you must define and enable a constraint that designates the department_id column of the departments table as a primary or unique key.

The foreign key constraint definition does not use the FOREIGNKEY clause, because the constraint is defined inline. The datatype of the department_id column is not needed, because Oracle automatically assigns to this column the datatype of the referenced key.

The constraint definition identifies both the parent table and the columns of the referenced key. Because the referenced key is the primary key of the parent table, the referenced key column names are optional.

Alternatively, you can define this foreign key constraint out of line:

Because of the first ONDELETE clause, if manager number 2332 is deleted from the employees table, then Oracle sets to null the value of manager_id for all employees in the dept_20 table who previously had manager 2332.

Because of the second ONDELETE clause, Oracle cascades any deletion of a department_id value in the departments table to the department_id values of its dependent rows of the dept_20 table. For example, if Department 20 is deleted from the departments table, then Oracle deletes all of the employees in Department 20 from the dept_20 table.

Composite Foreign Key Constraint Example

The following statement defines and enables a foreign key on the combination of the employee_id and hire_date columns of the dept_20 table:

The constraint fk_empid_hiredate ensures that all the employees in the dept_20 table have employee_id and hire_date combinations that exist in the employees table. Before you define and enable this constraint, you must define and enable a constraint that designates the combination of the employee_id and hire_date columns of the employees table as a primary or unique key.

The EXCEPTIONSINTO clause causes Oracle to write information to the wrong_emp table about any rows in the dept_20 table that violate the constraint. If the wrong_emp exceptions table does not already exist, then this statement will fail.

Check Constraint Examples

The following statement creates a offices table and defines a check constraint in each column of the table:

pk_od identifies the combination of the order_id and part_no columns as the primary key of the table. To satisfy this constraint, no two rows in the table can contain the same combination of values in the order_id and the part_no columns, and no row in the table can have a null in either the order_id or the part_no column.

fk_oid identifies the order_id column as a foreign key that references the order_id column in the orders table in the sample schema oe. All new values added to the column order_detail.order_id must already appear in the column oe.orders.order_id.

fk_pno identifies the product_id column as a foreign key that references the product_id column in the product_information table owned by oe. All new values added to the column order_detail.product_id must already appear in the column oe.product_information.product_id.

nn_qty forbids nulls in the quantity column.

check_qty ensures that values in the quantity column are always greater than zero.

check_cost ensures the values in the cost column are always greater than zero.

This example also illustrates the following points about constraint clauses and column definitions:

Out-of-line constraint definition can appear before or after the column definitions. In this example, the out-of-line definition of the pk_od constraint precedes the column definitions.

A column definition can contain multiple inline constraint definitions. In this example, the definition of the quantity column contains the definitions of both the nn_qty and check_qty constraints.

A table can have multiple CHECK constraints. Multiple CHECK constraints, each with a simple condition enforcing a single business rule, are preferable to a single CHECK constraint with a complicated condition enforcing multiple business rules. When a constraint is violated, Oracle returns an error identifying the constraint. Such an error more precisely identifies the violated business rule if the identified constraint enables a single business rule.

Attribute-Level Constraints Example

The following example guarantees that a value exists for both the first_name and last_name attributes of the name column in the students table: